Thanks JonMikal - to be honest, I couldn't really see the tilt myself (otherwise I would have corrected it! ) so I ran it through photoshop with the grid enabled. I'm still really struggling to find a horizontal/vertical reference point in the photo to compare to the grid, but I've given it a very slight (0.2º CCW) adjustment which brings the darkest band of cloud near enough horizontal on average. Any improvement?